This invention relates generally to a development apparatus for ionographic or electrophotographic imaging and printing apparatuses and machines, and more particularly is directed to an apparatus and method for enhancing toner charge level in a hybrid scavengeless development system.
Generally, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive surface is exposed to a light image from either a scanning laser beam or an original document being reproduced. This records an electrostatic latent image on the photoconductive surface. After the electrostatic latent image is recorded on the photoconductive surface, the latent image is developed. Two component and single component developer materials are commonly used for development. A typical two component developer comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. A single component developer material typically comprises toner particles. Toner particles are attracted to the latent image forming a toner powder image on the photoconductive surface, the toner powder image is subsequently transferred to a copy sheet, and finally, the toner powder image is heated to permanently fuse it to the copy sheet in image configuration.
The electrophotographic marking process given above can be modified to produce color images. One color electrophotographic marking process, called image on image processing, superimposes toner powder images of different color toners onto the photoreceptor prior to the transfer of the composite toner powder image onto the substrate. While image on image process is beneficial, it has several problems. For example, when recharging the photoreceptor in preparation for creating another color toner powder image it is important to level the voltages between the previously toned and the untoned areas of the photoreceptor.
Moreover, the viability of printing system concepts such as image on image processing usually requires development systems that do not scavenge or interact with a previously toned image. Since several known development systems, such as conventional magnetic brush development and jumping single component development, interact with the image receiver, a previously toned image will be scavenged by subsequent development, and as these development systems are highly interactive with the image bearing member, there is a need for scavengeless or noninteractive development systems.
Single component development systems use a donor roll for transporting charged toner to the development nip defined by the donor roll and photoconductive member. The toner is developed on the latent image recorded on the photoconductive member by a combination of mechanical and/or electrical forces. Scavengeless development and jumping development are two types of single component development systems that can be selected. In one version of a scavengeless development system, a plurality of electrode wires are closely spaced from the toned donor roll in the development zone. An AC voltage is applied to the wires to generate a toner cloud in the development zone. The electrostatic fields associated with the latent image attract toner from the toner cloud to develop the latent image. In another version of scavengeless development, interdigitated electrodes are provided within the surface of a donor roll. The application of an AC bias between the adjacent electrodes in the development zone causes the generation of a toner cloud. In jumping development, voltages are applied between a donor roll and the substrate of the photoreceptor member. In one version, only a DC voltage is applied to the donor roll to prevent toner deposition in the nonimage areas. In the image areas, the electric field from the closely spaced photoreceptor attracts toner from the donor. In another version, an AC voltage is superimposed on the DC voltage for detaching toner from the donor roll and projecting the toner toward the photoconductive member so that the electrostatic fields associated with the latent image attract the toner to develop the latent image.
A problem encountered with single component and hybrid development systems is low toner charge on the donor roll. Normal means to enhance charge in hybrid development systems is to change the toner/carrier formulations. In some cases this is not sufficient to ensure adequate toner charge on the donor roll. High triboelectric charge is necessary to overcome fringe field effects at latent image boundaries in the noninteractive image-on-image development systems. Attempts to enhance the charge via changes in the toner and carrier formulation have proven to be unsuccessful.
Briefly, the present invention obviates the problems noted above by providing an apparatus and method for enhancing toner charge level in a hybrid scavengeless development system. The system includes a housing defining a chamber storing a supply of developer material comprising toner; a toner donor member spaced from the surface and being adapted to transport toner to a region opposed from the surface; means for conveying said developer material in the chamber of said housing onto a region of said donor member; means for ion charging said toner loaded on the region of said donor member; and an electrode member space near the surface of said donor member, said electrode member being electrically biased by a power supply to detach toner from said donor member as to form a toner cloud for developing the latent image.